Devices for altering the propagation of electromagnetic radiation, such as by filtering and modulation, represent fundamental building blocks for many technological endeavors. Filtering refers generally to the selective treatment of electromagnetic radiation, such as selective transmission or reflection, based on frequency of the electromagnetic radiation. Modulation refers generally to the timewise variation of a property of an electromagnetic wave or signal, such as amplitude, frequency, phase, etc., according to a time varying control signal or modulation signal. Optical filtering and optical modulation refer to the filtering and modulation, respectively, of electromagnetic radiation at optical frequencies, which can include infrared, visible, and ultraviolet frequencies.
For certain electrooptical or all-optical applications there is sometimes a need for an optical filter exhibiting a very narrow reflection band centered near a specific frequency, while also exhibiting a flat, low-loss transmission band at other frequencies including nearby frequencies. By way of example, there may be a goal of separating out a particular mode from the output beam of a semiconductor laser source, and that mode might only be separated from nearby modes by as little as 1 nm or less (i.e., separated by a frequency difference corresponding to a free-space wavelength difference of 1 nm or less). A flat, low-loss transmission band can be particularly important if an application requires multiple such optical filters to be placed in optical series with each other.
With regard to optical modulation, the feasibility or desirability of a particular optical modulator for a particular application can often depend not only upon how well a target optical frequency range is modulated by that optical modulator, but also upon how well non-target frequencies are not modulated (or otherwise perturbed) by that optical modulator. Thus, for example, it may be desirable for an optical modulator to provide effective ON-OFF modulation for a first optical frequency f1 responsive to a modulation control signal, while allowing nearby optical frequencies f0 and f2 to pass through unperturbed, with little or no attenuation and no timewise relationship to the modulation control signal.
More generally, practical issues often arise in the implementation of at least one of optical filters and optical modulators in regard to one or more of modulation speed, frequency selectivity, spectral range of operation, noise performance, device cost, heat dissipation, device size, device tunability, and device power consumption. Other issues arise as would be apparent to one skilled in the art in view of the present disclosure.